The present invention relates to a process for the production of a coal-water mixture.
A coal-water mixture (abbreviated to CWM hereinbelow) can be transported through a pipe like liquid fuel and the mixture is widely used as a fuel for a boiler or a thermal power plant.
In the production of CWM, it is important that the coal is pulverized to yield a particle size distribution such that small coal particles fit into spaces among large coal particles. The processes for producing CWM are classified into the dry process, the wet process and the combined dry-wet process.
According to the dry process, pulverized coal particles differing from one another in the particle size, are produced by dry pulverization using a plurality of pulverizers. These particles are mixed together by controlling the mixing ratio so as to obtain a necessary particle size distribution. Water is added to the mixture and the mixture is kneaded to obtain CWM.
This process is advantageous in that the power cost for the pulverization is small because the pulverization is carried out in a dried state, but the pulverized coal shows such a strong water repellency and kneading thereof with water is relatively difficult, because drying is conducted at the same time with the pulverization. Therefore, the dry process is defective in that a long time and a large amount of power are necessary for the kneading operation.
According to the wet process, in order to eliminate the defect of the dry process, that is, the difficulty in kneading pulverized coal with water, water is added to the coal and pulverization and kneading are simultaneously carried out to attain the production of CWM at once.
However, in the wet process, since pulverization and kneading are simultaneously carried out, the pulverization process is slow and a long time is necessary for the completion of the operation. Furthermore, since large quantities of balls etc. must be used as tumbling agents to pulverize the coal, the power consumption for the pulverization drastically increases. Moreover, this process is disadvantageous over the dry process in that a complicated mill has to be used, the equipment cost increases and it is technically difficult to carry out the operation on a large scale.
Still further, the particle size adjustment for interposing smaller coal particles among coal particles, which is necessary for the production of a high-concentration slurry comprising fine particles of coal dispersed in water at a concentration of about 70%, is difficult in the wet process.
As means for overcoming these disadvantages, there has been proposed a two-step pulverizing method in which wet pulverization is carried out once at a relatively low concentration as the preliminary pulverization step and water is removed from the pulverization product before wet pulverization is carried out again to prepare CWM.
Although this two-step pulverization method attempts to mitigate the long pulverization time and large power consumption involved in the one-step pulverization method substantial benefits are not realized because it becomes necessary to add a dehydrating step prior to the second pulverization step.
The combined dry-wet process is an attempt to overcome the defects o both the dry and wet processes. According to this process, pulverized coal particles differing from one another in particle size are produced by both the dry and wet pulverization processes, and both the coal particles are combined together and kneaded to prepare CWM.
Although the problems of each of the dry and wet processes can be solved to some extent by the combined dry-wet process, the defects of the dry and wet processes remain to a certain extent.
Each of the three foregoing processes for the production of CWM has its own defects, and none of them has been established as an industrial process for the production of CWM.
Under these circumstances, manufacturers are now developing elaborate and unique processes and apparatus of their own.
For example, some of the present inventors proposed a process in which pulverized coal having a predetermined particle size, which has been obtained through the dry pulverization process and the particle size adjustment, is incorporated into a mixed-air jet pump (MJP) water stream to prepare CWM (see Japanese Patent Application Kokai Publication No. 62-223296).
In this process, pulverized coal in hot air, the particle size of which has been adjusted, is collected by gas-solid separation using a pulverized coal collector such as a bag filter, stored in a pulverized coal bin and introduced into an MJP water stream. Accordingly, this process is defective in that the equipment cost is relatively high and a large area is necessary for setting the bag filter.
Moreover, in an ordinary dry coal pulverizing mill, the quantity of hot air used for drying and classification of the coal is so large that the power consumption and equipment cost of fans cannot be neglected.
Furthermore, since the strong water repellency of the pulverized coal cannot be eliminated by the incorporation thereof into an MJP water stream, any homogeneous high-concentration slurry cannot be stably obtained.